fs/ext4: use try_cmpxchg in ext4_update_bh_state

Use try_cmpxchg instead of cmpxchg (*ptr, old, new) == old in
ext4_update_bh_state.  x86 CMPXCHG instruction returns success in ZF flag,
so this change saves a compare after cmpxchg (and related move instruction
in front of cmpxchg).

Also, try_cmpxchg implicitly assigns old *ptr value to "old" when cmpxchg
fails.  There is no need to re-read the value in the loop.

No functional change intended.

Link: https://lkml.kernel.org/r/[email protected]
Signed-off-by: Uros Bizjak <[email protected]>
Cc: "Theodore Ts'o" <[email protected]>
Cc: Andreas Dilger <[email protected]>
Signed-off-by: Andrew Morton <[email protected]>